1. Field of the Invention
The present invention relates generally to radiation-absorbing layers for thermopile radiation detectors and methods for fabricating thermopile radiation detectors.
2. Description of the Background Art
A thermopile includes a plurality of thermocouples that are connected in series. Each thermocouple relies on the generation of the well-known Thompson and Peltier emfs that result from a temperature gradient across a junction between two dissimilar metals. A combination of the Thompson and Peltier effects produces the Seebeck emf, which is used in thermocouples.
In general, a thermopile radiation detector has a first set of junctions (hot junctions) that make good thermal contact with a radiation receiver (e.g., a black body) but which are electrically insulated from the radiation receiver. A second set of junctions (cold junctions) is attached to a support, which does not receive the radiation and which is therefore at a lower temperature. The incident radiation raises the temperature of the radiation receiver and produces a voltage output from the thermopile that is proportional to the energy absorbed. That is, the thermopile output voltage is proportional to a temperature difference and is, therefore, proportional to the total radiation energy received.
Thermopile infrared detectors are known, which have been constructed on electrically insulating thermally conducting membranes. This approach generally increases the electrical response of the thermopile. However, the increase in electrical response is made at the expense of special and complex “Back-end” processes such as backside etching with anisotropic etches.
Reference can be made to U.S. Pat. No. 3,405,271 (issued Oct. 8, 1968) entitled “Detector Having Radiation Collector Supported on Electrically Insulating Thermally Conducting Film” by N. B. Stevens et al., and also to U.S. Pat. No. 3,405,272 (issued Oct. 8, 1968) entitled “Film Supported Detector with Low Heat Transfer Impedance Path from Cold Junctions to Thermal Sink” by N. B. Stevens et al. In both of these U.S. patents, an aluminum oxide is disposed over surfaces of a cold sink and spans a cavity within the cold sink. The aluminum oxide film supports two thermoelectric materials (Bi and Sb).
A radiation-absorbing layer of blackening material is positioned locally within a small region that includes a thermocouple junction. The blackening material, such as gold (Au) black or bismuth (Bi) black, is evaporated on to the small region. In U.S. Pat. No. 3,405,272, a porous layer of bismuth (Bi) material is positioned over the surface of each collector. The blackening material renders the surface absorptive, thus increasing the electrical response (in volts output per watt of radiation received) of the thermopile.
As a radiation-absorbing layer for a thermopile infrared detector, gold (Au) is evaporated to grow dendrites on the thermocouple surface. A suggested method of growing dendrites of gold (Au) is to position the thermopile within a vacuum chamber so that it may be physically moved from the outside of the chamber. The majority of air is evacuated from the vacuum chamber down to a predetermined pressure. Subsequently, inertia gas is fed at a predetermined pressure to fill the vacuum chamber, keeping the system pressure within the chamber constant at a vacuum level falling in a range over several hundreds Pa. At such vacuum level, the mean free path within the vacuum chamber is extremely short. Gold material is positioned in an evaporation port directly below the thermocouple surface. The gold is heated and a gold smoke is produced which rises and engages the surface. Strict control of the evaporation rate is needed to allow dendrites to grow on the thermocouple surface.
The radiation-absorbing layer of dendrites of gold exhibits a high degree of absorptance exceeding 90%. However, one drawback to the use of dendrites of gold (Au) in thermopile detectors is a lack of a suitable patterning process that is compatible with standard integrated circuit processing techniques. Besides, the dendrites, which are unsuitable for conventional etching, require special care in a patterning process to remove unnecessary portions leaving the thermocouple surface.
As a radiation-absorbing layer for a thermopile infrared detector, a non-porous multi-layered construction is provided, which achieves a high degree of absorption by trapping radiation within a resonant cavity. This multi-layered radiation-absorbing layer can be formed by the standard integrated circuit processing techniques. However, one disadvantage of this conventional multi-layered construction is that its absorptance is about 70% and subject to variations with different wavelengths of the incident radiation. Total energy absorbed is therefore small. The use of this conventional approach is limited to the case where the electrical response (in volts output per watt of radiation received) of thermopile detectors is high.
An object of the present invention is to provide radiation-absorbing layers for thermopile radiation detectors, and methods for fabricating thermopile radiation detectors, that overcome the foregoing and other problems.
Another object of the present invention is a radiation-absorbing layer that is formed using conventional semiconductor processing techniques including photolithography and etching.
Another object of the present invention is to provide a radiation-absorbing layer that highly absorbs electromagnetic radiation within a desired band of wavelengths.
A further object of the present invention is to provide a radiation-absorbing layer that selectively absorbs electromagnetic radiation within an infrared band of wavelengths.
A still further object of the present invention is to provide a radiation-absorbing layer that absorbs electromagnetic radiation within a predetermined band of wavelengths and reflects electromagnetic radiation outside of the predetermined band of wavelengths.